The present invention relates to an apparatus for pulverizing rare-earth alloy magnetic materials through absorption and release of hydrogen (in this specification, such an apparatus will be called a xe2x80x9chydrogen pulverizerxe2x80x9d) The present invention also relates to respective methods for preparing rare-earth alloy magnetic material powder and producing a magnet using the hydrogen pulverizer.
A rare-earth sintered magnet is produced by pulverizing a magnetic alloy into alloy powder, pressworking and sintering the alloy powder and then subjecting the sintered alloy to aging treatment. Two types of rare-earth alloy magnets, namely, samarium-cobalt (Smxe2x80x94Co) magnets and neodymium-iron-boron magnets, are used widely in various applications. In this specification, a rare-earth alloy magnet of the latter type will be referred to as an xe2x80x9cRxe2x80x94Txe2x80x94(M)xe2x80x94B magnetxe2x80x9d, where R is a rare-earth element including Y, T is Fe or a compound of Fe and at least one transition metal element, M is an additive and B is boron. Part of Fe in an Rxe2x80x94Fexe2x80x94B type magnet can be replaced with a transitional metal element, e.g., cobalt. The Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnet is often applied to many kinds of electronic units, because the maximum energy product thereof is the higher than any other kind of magnet and yet the cost thereof is relatively inexpensive.
In a conventional process of pulverizing material alloy for the Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnet, a container made of stainless steel like SUS304 is loaded with the magnetic material alloy powder and then primary pulverization of the material alloy is carried out in a hydrogen furnace, where hydrogen is absorbed and released into/out of the material alloy.
Methods for preparing the rare-earth alloy are roughly classified into the following two types. The first type is an ingot mold casting technique, in which a melt of material alloy is teemed into a mold and then cooled down relatively slowly. The second type is a quenching technique, such as a strip-casting process or a centrifugal casting process, in which a melt of material alloy is rapidly quenched by a single roll, twin rolls, a rotating disk, or a rotating cylinder, thereby forming, out of the molten alloy, a solidified alloy, which is thinner than the alloy produced by the conventional ingot mold casting technique.
According to the quenching technique, the thickness of the resultant Rxe2x80x94Txe2x80x94(M)xe2x80x94B magnet alloy is in the range from 0.03 mm to 10 mm, both inclusive. The molten alloy starts to solidify from the surface that has come into contact with the chill roll or its equivalents, and subsequently columnar crystals are growing from the surface in the thickness direction. As a result, the quenched alloy comes to have a structure including R2T14B crystal grains and R-rich phases that exist dispersively along the R2T14B crystal grain boundaries. The sizes of the R2T14B crystal grain are in the range from 0.1 xcexcm to 100 xcexcm, both inclusive, in the minor axis direction and in the range from 5 xcexcm to 500 xcexcm, both inclusive, in the major axis direction. The R-rich phases are non-magnetic phases in which the concentration of the rare-earth element R is relatively high. The thickness of the R-rich phases, which corresponds to the width of the grain boundaries, is 10 xcexcm or less.
Compared to an ingot alloy, i.e., alloy that has been prepared by the conventional mold casting process (i.e., die casting process), the quenched alloy has been cooled down in a relatively short period of time. Thus, the crystal structure or the grain size of the quenched alloy is finer than that of the ingot alloy. That is to say, the grain boundaries of the quenched alloy are greater in area, and the R-rich phases exist in the grain boundaries. Accordingly, the quenched alloy is also superior to the ingot alloy in terms of dispersiveness of the R-rich phases.
The quenched alloy is likely to fracture at the grain boundaries during a hydrogen pulverizing process. For that reason, the R-rich phases easily appear on the surface of the alloy powder particles that are obtained by pulverizing the quenched alloy. In the R-rich phases, R easily reacts with oxygen. Accordingly, the quenched alloy powder is very likely to be oxidized, generate heat and spontaneously ignite. Thus, it is believed that the magnetic properties of the strip-cast alloy powder are deteriorative considerably.
Next, a known hydrogen pulverization process for the ingot alloy will be described.
First, a process container in the shape of a flat pack is filled with magnetic alloy blocks (each having a length of about 3 cm on each side) that have been cast in a water-cooled casting die, and then loaded into a rack. After the rack has been inserted into a hydrogen furnace, the pressure inside the furnace is reduced using a vacuum pump. Then, hydrogen gas is supplied into the hydrogen furnace, thereby getting hydrogen absorbed into the material alloy. After a predetermined time has passed, the material alloy is heated while evacuating the hydrogen furnace again, thereby getting hydrogen released from the material alloy. Once a sufficient quantity of hydrogen has been released from the material alloy and the alloy has been cooled down, the cap of the hydrogen furnace is opened and the rack, which is loaded with the process containers, is ejected to the air. At the point in time that the hydrogen pulverization process is finished, the alloy has been roughly broken up to a size of about 1 cm. Thereafter, the material, which has been pulverized roughly through this hydrogen process, is taken out of the container, ground finely to a size of about 10 xcexcm to about 400 xcexcm using a disk mill and then pulverized even more finely to an average particle size of about 2 xcexcm to about 5 xcexcm using a jet mill, for example.
A green compact (or as-pressed compact) is formed, by compaction, out of the material alloy fine powder prepared this way. Thereafter, the compact is subjected to sintering, aging treatment and so on to produce a sintered magnet.
According to the conventional process, however, resulting magnetic properties deteriorate. This is because when the material is ejected out of the hydrogen furnace to the air, the rare-earth element R contained in the hydrogen-pulverized material is oxidized due to the contact with the air.
Suppose the source material contains neodymium as the rare-earth element R, for example. In such a case, NdH3 is formed by getting hydrogen absorbed into the material, while NdH3 changes into NdH2 by getting hydrogen released from the material. In an actual mass production process, however, hydrogen cannot be released completely, and NdH3 is almost always left in part of the material. At the core of the process container, in particular, plenty of NdH3 might be left because the core cannot always be heat-treated sufficiently. If NdH3 remains in the material, then that NdH3 is exposed to the air to generate heat when the material is ejected out of the process container. Accordingly, in practice, a cooling period should be provided after the material has been taken out. In other words, the fine pulverization and other subsequent process steps cannot be started immediately. What is worse, there is a risk of spontaneous ignition.
We found that the probability of heat generation and spontaneous ignition due to oxidation is remarkably high when the hydrogen pulverization process is applied to the quenched alloy produced by the quenching technique (e.g., the strip-cast process), in particular. Thus, we concluded that it is extremely difficult to realize an industrialized quenched alloy pulverization process according to the conventional technique. Hereinafter, this point will be detailed.
Compared to the ingot alloy, the quenched alloy is thinner and has a finer metal structure. Accordingly, most of the quenched alloy has already been pulverized sufficiently (e.g., with an average size of 1.0 mm or less) when the hydrogen pulverization process on the alloy is over. Thus, the total surface area of the pulverized alloy is greater. Also, since R-rich phases exist with high dispersiveness, the R-rich phases are likely to appear on the surface of the hydrogen-pulverized powder. For these reasons, a large quantity of unreacted, active rare-earth element R is exposed on the surface of the strip-cast alloy powder that has just been subjected to the hydrogen pulverization process, and is very likely to be oxidized. Accordingly, there is a risk of spontaneous ignition unless the as-pulverized powder is cooled down to room temperature (i.e., about 20xc2x0 C.). Also, if the large quantity of rare-earth element exposed is oxidized or nitrided, the magnetic properties of a final magnet product are deteriorative considerably.
Even if the hydrogen-pulverized powder is cooled down within t he furnace using an inert gas at a low temperature to suppress such oxidation and nitriding reactions, some problems still happen. Specifically, when the cap of the furnace is opened, condensation is produced inside the furnace in such a case. As a result, vacuum pumping for the next lot will take a long time, because the water vaporizes inside the furnace. In addition, since the quenched alloy is pulverized into particularly fine powder, the as-pulverized alloy powder is hard to ventilate. That is to say, it is difficult for the cooling inert gas to remove sufficient heat from the pulverized powder, thus taking an adversely long time to cool the powder down and ultimately decreasing the productivity considerably.
object of the present invention is providing a hydrogen pulverizer that can perform the hydrogen pulverization and subsequent cooling processes more efficiently and safely with the total processing time shortened.
Another object of the present invention is providing a hydrogen pulverizer that can contribute to improvement in magnetic properties of a resultant magnet by preventing the material from being oxidized.
Still another object of the present invention is providing respective methods for preparing rare-earth alloy magnetic material powder and producing a magnet, by which the pulverization process can be carried out more efficiently and safely even on a rapidly-quenched alloy with a fine structure such as a strip-cast alloy.
An inventive hydrogen pulverizer is an apparatus for subjecting a rare-earth alloy magnetic material to a hydrogen pulverization process. The apparatus includes: a hermetically sealable hydrogen furnace, which includes a furnace body with an opening and a cap for closing the opening; a loading chamber for temporarily enclosing the rare-earth alloy magnetic material when the rare-earth alloy magnetic material, which has been pulverized with hydrogen, is unloaded from the furnace body through the opening; and means for supplying an inert gas into the loading chamber.
In one embodiment of the present invention, the cap of the hydrogen furnace may move inside the loading chamber to open or close the opening of the furnace body.
Alternatively or additionally, the loading chamber may include a door, and when the door is closed, a substantially airtight condition is created within the loading chamber.
In an alternate embodiment, the apparatus may further include a cooling system for supplying, into the hydrogen furnace, the inert gas at room temperature and the inert gas that has been cooled down in this order.
An inventive rotary cooler includes: a cooling cylinder supported in a freely rotatable position; cooling means for cooling down the cooling cylinder; control means for controlling the number of revolutions per minute of the cooling cylinder; and temperature sensing means provided for the cooling cylinder. The control means controls the number of revolutions per minute of the cooling cylinder based on the output of the temperature sensing means.
An inventive method for pulverizing a rare-earth alloy magnetic material with hydrogen is carried out by using an apparatus including: a hermetically sealable hydrogen furnace, which includes a furnace body with an opening and a cap for closing the opening; a loading chamber for temporarily enclosing the rare-earth alloy magnetic material when the rare-earth alloy magnetic material, which has been pulverized with hydrogen, is unloaded from the furnace body through the opening; and means for supplying an inert gas into the loading chamber.
An inventive method for preparing a rare-earth alloy magnetic material powder includes the step of pulverizing a rare-earth alloy magnetic material with hydrogen by using an apparatus. The apparatus includes: a hermetically sealable hydrogen furnace, which includes a furnace body with an opening and a cap for closing the opening; a loading chamber for temporarily enclosing the rare-earth alloy magnetic material when the rare-earth alloy magnetic material, which has been pulverized with hydrogen, is unloaded from the furnace body through the opening; and means for supplying an inert gas into the loading chamber. The method further includes the step of unloading the rare-earth alloy magnetic material from the apparatus and moving the material into an inert gas environment while supplying the inert gas into the loading chamber of the apparatus.
In one embodiment of the present invention, the method may further include the step of receiving the rare-earth alloy magnetic material that has been unloaded from the apparatus and then transporting the material using a transporter including means for supplying the inert gas into the transporter itself.
Alternatively or additionally, the method may further include the step of cooling down the rare-earth alloy magnetic material that has been pulverized with hydrogen by supplying the inert gas into the hydrogen furnace of the apparatus.
In this particular embodiment, the inert gas supplied into the hydrogen furnace of the apparatus is preferably circulated and used cyclically.
More specifically, the material is preferably cooled down to a predetermined temperature using, as the inert gas supplied into the hydrogen furnace of the apparatus, a cooled inert gas and then further cooled down using an inert gas at about room temperature.
In another embodiment of the present invention, the method may further include the step of unloading the rare-earth alloy magnetic material from the transporter inside a housing that is filled with the inert gas.
In still another embodiment, the method may further include the step of cooling down the rare-earth alloy magnetic material inside a cooling system that is filled with the inert gas.
An inventive method for producing a magnet includes the steps of: pulverizing a rare-earth alloy magnetic material using the apparatus according to the present invention; unloading the rare-earth alloy magnetic material from the apparatus and moving the material into the loading chamber filled with the inert gas; transporting the rare-earth alloy magnetic material that has been unloaded from the apparatus using a transporter including means for supplying the inert gas into the transporter itself; unloading the rare-earth alloy magnetic material from the transporter inside a housing that is filled with the inert gas, and cooling down the rare-earth alloy magnetic material inside a cooling system that is filled with the inert gas; making fine powder of the rare-earth alloy magnetic material by further pulverizing the rare-earth alloy magnetic material; and producing a magnet by compacting and sintering the fine powder of the rare-earth alloy magnetic material.
In one embodiment of the present invention, the method may further include the step of cooling down the rare-earth alloy magnetic material that has been pulverized with hydrogen by supplying the inert gas into the hydrogen furnace of the apparatus.
In this particular embodiment, the inert gas supplied into the hydrogen furnace of the apparatus is preferably circulated and used cyclically.
Alternatively or additionally, the material may be cooled down to a predetermined temperature using, as the inert gas supplied into the hydrogen furnace of the apparatus, a cooled inert gas and then further cooled down using an inert gas at about room temperature.
Another method for preparing a rare-earth alloy magnetic material powder according to the present invention includes the step of embrittling a rare-earth magnetic material alloy within a furnace with hydrogen supplied into the furnace. The alloy contains: R2T14B crystal grains, where R is a rare-earth element, T is Fe or a compound of Fe and at least one transition metal and B is boron; and R-rich phases existing dispersively in grain boundaries of the R2T14B crystal grains. The sizes of the R2T14B crystal grains are in the range from 0.1 xcexcm to 100 xcexcm, both inclusive, in a minor axis direction and in the range from 5 xcexcm to 500 xcexcm, both inclusive, in a major axis direction. The thickness of the alloy is in the range from 0.03 mm to 10 mm, both inclusive. The method further includes the step of unloading the alloy from the furnace within an inert gas environment.
Still another method for preparing a rare-earth alloy magnetic material powder according to the present invention includes the step of embrittling a rare-earth magnetic alloy within a furnace with hydrogen supplied into the furnace. The rare-earth magnetic alloy has been prepared by rapidly quenching a molten alloy to a thickness in the range from 0.03 mm to 10 mm, both inclusive, such that R2T14B crystal grains, where R is a rare-earth element, T is Fe or a compound of Fe and at least one transition metal element and B is boron, have grown in the alloy in the thickness direction thereof. The method further includes the step of unloading the alloy from the furnace within an inert gas environment.
In one embodiment of the present invention, the method may further include the steps of: cooling down the alloy, which has been embrittled with hydrogen, within the furnace; and moving the alloy, which has been unloaded from the furnace, into a cooling system and cooling down the alloy within the cooling system.
In this particular embodiment, the method preferably further includes the step of introducing the alloy into a process container and loading the container into the furnace before the alloy is embrittled with hydrogen. In the step of unloading the alloy from the furnace, the process container is preferably unloaded from the furnace within the inert gas environment, and the alloy is preferably cooled down within the cooling system after having been taken out of the process container.
In another embodiment of the present invention, the inert gas environment may be argon or helium gas environment.
In an alternative embodiment, the method may further include the step of cooling down the alloy within an inert gas environment after the alloy has been unloaded from the furnace.
In still another embodiment, the alloy may be cooled down while being stirred up within the inert gas environment.
Yet another method for preparing a rare-earth alloy magnetic material powder according to the present invention includes the step of embrittling a rare-earth magnetic alloy within a furnace with hydrogen supplied into the furnace. The rare-earth magnetic alloy has been prepared by rapidly quenching a molten alloy to a thickness in the range from 0.03 mm to 10 mm, both inclusive, such that R2T14B crystal grains, where R is a rare-earth element, T is Fe or a compound of Fe and at least one transition metal element and B is boron, have grown in the alloy in the thickness direction thereof. The method further includes the step of unloading the alloy from the furnace and cooling the alloy down within a cooling system while stirring the alloy up within an inert gas environment.
In one embodiment of the present invention, the cooling system may include a cylindrical member that is driven to rotate, and the number of revolutions per minute of the cylindrical member may be controlled based on the output of means for sensing the temperature of the alloy.
Another inventive method for producing a magnet includes the step of embrittling a rare-earth magnetic material alloy within a furnace with hydrogen supplied into the furnace. The alloy contains: R2T14B crystal grains, where R is a rare-earth element, T is Fe or a compound of Fe and at least one transition metal and B is boron; and R-rich phases existing dispersively in grain boundaries of the R2T14B crystal grains. The sizes of the R2T14B crystal grains are in the range from 0.1 xcexcm to 100 xcexcm, both inclusive, in a minor axis direction and in the range from 5 xcexcm to 500 xcexcm, both inclusive, in a major axis direction. The thickness of the alloy is in the range from 0.03 mm to 10 mm, both inclusive. The method further includes the steps of: unloading the alloy from the furnace within an inert gas environment; compacting powder of the alloy; and sintering the compacted alloy.
Still another inventive method for producing a magnet includes the step of embrittling a rare-earth magnetic alloy within a furnace with hydrogen supplied into the furnace. The rare-earth magnetic alloy has been prepared by rapidly quenching a molten alloy to a thickness in the range from 0.03 mm to 10 mm, both inclusive, such that R2T14B crystal grains, where R is a rare-earth element, T is Fe or a compound of Fe and at least one transition metal element and B is boron, have grown in the alloy in the thickness direction thereof. The method further includes the steps of: unloading the alloy from the furnace within an inert gas environment, compacting powder of the alloy; and sintering the compacted alloy.